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A Simple Method for Servo Motor Tuning

Primary Software:

Primary Software Version: 7.6

Primary Software Fixed Version: N/A

Secondary Software: N/A

Hardware: Motion Control>>Controllers>>PCI-7350

Problem:

I can't seem to get my servo motor tuned. Is there a simple method to find PID gains that will give me

something close to my desired response?

Solution:

This document is intended to complement the Understanding Servo Tune Developer Zone

document, which more thoroughly explores the concept of servo tuning. There is no single proper

way to do manual servo tuning, but the method outlined below provides a simple method to quickly

determine gains that are in a reasonable range to provide a good response from the system.

Tuning Features of Measurement & Automation Explorer

One way to tune an NI-Motion system is to open up the Step Response and Control Loop tabs as

separate windows. To do this, follow these steps:

1. In Measurement & Automation Explorer (MAX), go to Calibration»Servo Tune for your motion

controller.

2. Click and hold the Step Response tab

3. Drag the mouse until the Step Response tab appears as a separate window

4. Click the push pin icon at the top right of the window to keep the Step Response window

open.

5. Repeat these steps for the Control Loop tab. Arrange and resize the windows so that you can

see them side by side as you tune your motor. Images of the windows are shown below for

your reference.

Hello Can (This is not me)

The Method of Tuning

Now that you have the windows arranged so that you can easily modify your control loop, and see the

changes in the step response, take the following steps to tune your PID control system.

1. Set all three PID parameters, Kp, Kd, and Ki to 0

2. Start by tuning Kp. Set it to a number that is much lower than needed. If you are unsure, start

with 1.

3. Click the Step Response button to view a step response graph of your system. Step

response plots are described in further detail in the Understanding Servo Tune document

mentioned earlier.

4. If the graph shows the parameter is:

Too Low - Double the value of the parameter.

Too High - Set the parameter to halfway between the current value and the previous value.

5. Repeat steps 3 and 4 until you achieve a reasonable value for Kp. For most systems, this will

mean that the response will approach the input, and oscillate continually about the input with

a small amount of dampening. If the oscillation does not gradually decrease in amplitude as

shown below, then the system is considered unstable. If this occurs, you may need to add a

small amount of Kd while you are repeating steps 3 and 4 to increase Kp.

6. After you arrive at a reasonable value for Kp, move on to Kd.

7. Repeat steps 3 and 4 for Kd until you achieve a reasonable value for Kd. For most systems,

this will mean that the response will no longer oscillate continually, but will quickly dampen to

a steady state value. This steady state value may be slightly offset from the input value, and

this offset can be corrected with an appropriate Ki value.

8. After you arrive at a reasonable value for Kd, move on to Ki.

9. Repeat steps 3 and 4 for Kd until you achieve a reasonable value for Ki. This parameter

works on the integral of the position error therefore taking out offset error. Please use this

parameter conservatively as it can introduce instability into the system.

A Tuning Example

The table below provides an example of using this method to quickly tune a servo motor. This

example took 20 iterations to arrive at a reasonably well tuned system. The gains used in each

iteration are shown as well as the step response graph and characteristics. The screenshots from

this tuning are also shown in the animated image of the Step Response window at the beginning of

this document.

Step Kp Kd Ki

Settling

Time

(ms)

Rise

Time

(ms)

Peak

Time

(ms)

Max

Overshoot

(%)

Step Response

1 1 0 0 315 96 165 40

2 2 0 0 363 57 114 67

3 4 0 0 567 39 81 89

4 8 0 0 594 27 60 91

5 8 1 0 594 27 60 88

6 8 2 0 594 30 63 88

7 8 4 0 594 27 60 88

8 8 8 0 588 30 63 84

9 8 16 0 501 27 60 78

10 8 32 0 366 30 60 68

11 8 64 0 255 30 60 53

12 8 128 0 162 33 60 29

13 8 256 0 105 45 75 3

14 8 192 0 132 39 66 13

15 8 192 1 534 42 66 3

16 8 192 2 582 36 69 28

17 8 192 4 561 36 69 27

18 8 192 8 237 33 66 54

19 8 192 16 462 30 63 75

20 8 192 12 546 30 63 63

Related Links:

Developer Zone: Understanding Servo Tune

KnowledgeBase 39DHP4QX: Unable to Tune a Servo Motor

KnowledgeBase 1A1FUIYP. Torque and Velocity Mode Differences and NI-Motion Compatibility

National Instruments Motion Control Support Homepage

Attachments:

Report Date: 14.06.2007

Last Updated: 05.03.2012

Document ID: 4AD9N5P9

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